The invention relates to a hydraulic motor vehicle brake system with electronic anti-lock control, with a pedal actuated brake pressure transducer, with electrically actuated pressure medium inlet valves and outlet valves inserted into the pressure medium paths, by which the inflow of pressure medium into the wheel brakes of the individual wheels and the pressure medium drainage into a pressure equalization reservoir of the brake system are individually controlled, with wheel sensors to determine the rotational relationship of the wheels, as well as with an electronic control circuit to evaluate the sensor signals, to determine the driving conditions and to generate the brake pressure control signals.
A circuit arrangement for a brake system with electronic anti-lock control is also a constituent of the invention.
anti-lock braking systems (ABS) are available in many different types. Hydraulic systems in which the wheel brakes of the individual wheels are connected to a multi-circuit brake pressure transducer or master cylinder by means of electrically actuated hydraulic valves have proven themselves to be of particular value. An inlet valve is located in the pressure medium path from the master cylinder to the wheel brake, switched to through-flow when in its normal position, while it switches to the blocking position when in the constant pressure phase or in the pressure reduction phase. An outlet valve blocked when in the resting position is located in the return flow line which leads from the wheel brake to the low pressure reservoir or--in open systems--to the pressure equalization reservoir of the brake system. To return the pressure medium drained off during the pressure reduction phase, depending on the system, a return flow pump (in closed systems) or a hydraulic pump that advances fluid from the pressure equalization reservoir into the brake circuit (in open systems) is used.
The reduction of manufacturing expenses for such anti-lock braking systems (ABS) resulting from the simplification of the system and its components, from skillful construction of the hydraulic brake circuit, from the elimination of components and from the transfer of functions to the allocated electronics has been the object of intensive investigation for some considerable time. These efforts are fundamentally constrained by the fact that a brake system is also expected to perform with the highest reliability even under unfavorable, seldom arising circumstances. An "exhaustion" of the pressure medium volume required for effective braking--as a consequence of frequent, long lasting pressure reductions on a smooth road or under rapidly changing road conditions must naturally be avoided in order to maintain proper brake operation.
An important cost factor for such anti-lock systems is represented by the hydraulic pump and the associated electro-motor drive. It is therefore now recommended that a pump-less ABS be developed. An example of this is the brake system known from DE 31 09 372 A1, which provides that pressure medium drained off by means of outlet valves in order to regulate the anti-lock protection be collected in a low pressure reservoir, from where, after releasing the brakes, it can then flow back into the brake circuit. This known system contains two brake circuits with a diagonal brake circuit subdivision, whereby each brake circuit is equipped with a common inlet/outlet valve arrangement and additionally with a further, electrically switchable inlet valve which is inserted in the pressure medium path leading to the rear wheel brake. This inlet valve can, on the one hand, be switched into a blocking position interdependent with the anti-lock control, and on the other hand interdependent with the load. Completely independent control of all wheels, however, is not provided.
Furthermore, a two circuit hydraulic brake system with anti-lock control is known from DE 43 34 838 A1, by which the pressure medium that is led away from the wheel brakes in the brake pressure reduction phase reaches the low pressure reservoirs, which have two separated inlets for the connection of a rear wheel and a front wheel. Upon achieving a predetermined degree of filling of the reservoir the pressure medium inflow from the front wheel brake, and thereby a further pressure reduction on the front brake, is blocked. Up to this point in time there are hardly any limits on the ABS operation, although the volume through-flow is drastically reduced with the help of specific control algorithms. When a locking of the front wheel is sensed, a switch-over of the control algorithm for the rear wheel is carried out, and a locking of the rear axle is avoided (safety control mode). Maintenance of the brake function is assured by the limited volume uptake of the low pressure reservoir.
The present invention now has the basic object of developing a pump-less ABS of this type, that functions without the use of a low pressure reservoir and that, even under unfavorable circumstances, still features the desired high reliability of operation.